The present invention relates to improved resin compositions for peel-off coatings and a method for the preparation of same.
Peel-off coatings formed by a coating composition serve to protect the surfaces of various shaped articles. For example, the peel-off coatings applied to the surfaces of an electric part, machine part or the like are intended to protect those surfaces from being soiled or mechanically or chemically damaged during storage or shipment. Such coatings are subjected to peeling after the storage or shipment and just before actual use of the article, so that the intrinsic surfaces will be exposed for proper functioning or operation.
As a further example, the peel-off coatings applied to the surfaces of an article which is for use in an environment liable to cause heavy staining or soiling, such as ventilation fans used in a cooking room, can be peeled off as and when stained to such an extent as to necessitate cleaning, so that the exposed surfaces are ready for recoating. This leads to a saving of cleaning labor much more than otherwise.
The above peel-off coatings have not been found to be always satisfactory to meet the following requirements: (1) that coating films should be maintained completely adherent to substrate throughout their life, i.e., from formation to peel-off, (2) that coating films should have a stable peelability, (3) that coating films as the protective coating should have sufficiently high mechanical strengths and resistance to water and chemicals, (4) that the peelability and strengths of coating films should be good even at elevated temperatures, (5) that coating can be carried out either by brushing or spraying, and (6) that coating compositions should have a high drying velocity.
One typical example of the coating compositions to form the peel-off coatings, which is of the aqueous emulsion type and most widely used, is a polyvinyl acetate emulsion or an acrylic resin emulsion. These emulsions are defective with respect of heat stability, resistance to water, and stability in storage. For example, those disclosed in Japanese Patent Publication No. 14770/73, which are prepared by adding floury materials, such as calcium carbonate and titanium dioxide, silicone resins, and an aqueous emulsion of paraffin wax, are defective in their poor stability when stored for a long time and also in their low drying velocity after application.
Besides the above aqueous emulsion type coating compositions, solution-type compositions are also known, which are prepared by dissolving a polyvinyl butyral resin and a plasticizer in an organic solvent. Those compositions are disadvantageous due to the restrictions in the selection and amount of the plasticizer to be used and also the possibility of bleeding or migration occurring to the plasticizer used. In addition, the resultant coating films sometimes exhibit a low peelability when provided on surfaces of certain materials.
Further, it should be added that the inventors of the present invention disclosed an improved similar peel-off coating composition which comprises a polyvinyl acetal resin as the film-forming resin material and an organopolysiloxane having a polyoxyalkylene group as the side chain in their Copending U.S. Patent Application Ser. No. 699,598, filed June 24, 1976. This composition, however, has been found to be still somewhat disadvantageous due to the fact that the resultant coating films exhibit rather a poor peelability after prolonged heating at an elevated temperature of, say, 60.degree. C. or higher.